The present invention relates to cover layers for golf balls incorporating material compositions having relatively small amounts of sound-altering materials mixed therein, such that sound produced by the golf balls when struck is selectively altered, while the mechanical characteristics of the covers remain substantially the same. The present invention also relates to methods of manufacture of golf ball covers incorporating these sound-altering materials.
Golf balls generally include a core and at least one cover layer surrounding the core. Balls can be classified as two-piece, multi layer, or wound balls. Two-piece balls include a spherical inner core and an outer cover layer. Multi-layer balls include a core, a cover layer and one or more intermediate (or mantle) layers. The intermediate layers themselves may include multiple layers. Wound balls include a core, a rubber thread wound under tension around the core to a desired diameter, and a cover layer, typically of balata material or thermoset polyurethane.
Generally, two-piece balls provide good durability and ball distance when hit, but they provide poor ball control, due to low spin rate and poor “feel” (the overall sensation transmitted to the golfer while hitting the ball). Wound balls having balata covers generally have high spin rate, leading to good control, and good feel, but they have short distance and poor durability in comparison to two-piece balls. Multi-layer balls generally have performance characteristics between those of two-piece and wound balls. Multi-layer balls exhibit distance and durability inferior to two-piece balls but superior to wound balata, and they exhibit feel and spin rate inferior to wound balata and thermoset polyurethane balls but superior to two-piece balls. Thermoset polyurethane covers tend to have very good durability, but they have not yet attained the preferred feeling of balata.
Material characteristics of the compositions used in the core, cover, and any intermediate layers are important in determining the performance of the resulting golf balls. In particular, the composition of the cover layer is important in determining the ball's durability, scuff resistance, speed, shear resistance, spin rate, feel, and “click” (the sound made when a golf club head strikes the ball). Various materials having different physical properties are used to make cover layers to create a ball having the most desirable performance possible. For example, many modern cover layers are made using soft or hard ionomer resins, elastomeric resins or blends of these. Ionomeric resins used generally are ionic copolymers of an olefin and a metal salt of an unsaturated carboxylic acid, or ionomer terpolymers having a co-monomer within its structure. These resins vary in resiliency, flexural modulus, and hardness. Examples of these resins include those marketed under the name SURLYN manufactured by E.I. DuPont de Nemours & Company of Wilmington, Del., and IOTEK manufactured by ExxonMobil Corporation of Irving, Tex. Elastomeric resins used in golf ball covers include a variety of thermoplastic or thermoset elastomers available. Layers other than cover layers also significantly affect performance of a ball. The composition of an intermediate layer is important in determining the ball's spin rate, speed, and durability. The composition and resulting mechanical properties of the core are important in determining the ball's coefficient of restitution (C.O.R.), which affects ball speed and distance when hit. In addition to the performance factors discussed above, processability also is considered when selecting a formulation for a golf ball composition. Good processability allows for ease of manufacture using a variety of methods known for making golf ball layers, while poor processability can lead to avoidance of use of particular materials, even when those materials provide for good mechanical properties.
Various materials having different physical properties are used to make ball layers to create a ball having the most desirable performance possible. Each of the materials discussed above has particular characteristics that can lead to ball properties when used in a golf ball composition, either for making a ball cover, intermediate layer, or core. However, one material generally cannot optimize all of the important properties of a golf ball layer. Properties such as feel, speed, spin rate, resilience and durability all are important, but improvement of one of these properties by use of a particular material often can lead to worsening of another. For example, ideally, a golf ball cover should have good feel and controllability, without sacrificing ball speed, distance, or durability. Despite the broad use of copolymeric ionomers in golf balls, their use alone in, for example, a ball cover can be unsatisfactory. A cover providing good durability, controllability, and feel would be difficult to make using only a copolymeric ionomer resin having a high flexural modulus, because the resulting cover, while having good distance and durability, also will have poor feel and low spin rate, leading to reduced controllability of the ball. Also, the use of particular elastomeric resins alone can lead to compositions having unsatisfactory properties, such as poor durability and low ball speed.
Therefore, to improve golf ball properties, the materials discussed above can be blended to produce improved ball layers. Prior compositions for golf balls have involved blending high-modulus copolymeric ionomer with, for example, lower-modulus copolymeric ionomer, terpolymeric ionomer, or elastomer. As discussed above, ideally a golf ball cover should provide good feel and controllability, without sacrificing the ball's distance and durability. Therefore, a copolymeric ionomer having a high flexural modulus often is combined in a cover composition with a terpolymeric ionomer or an elastomer having a low flexural modulus. The resulting intermediate-modulus blend possesses a good combination of hardness, spin and durability.
However, even with blending of materials to improve ball properties, use of the materials and blends discussed above has not been completely satisfactory. Improving one characteristic can lead to worsening of another. For example, blending an ionomer having a high flexural modulus with an ionomer having a low flexural modulus can lead to reduced resilience and durability compared to use of the high-modulus ionomer alone. Also, the hardness of the compositions that can be obtained from these blends are limited, because durability and resilience get worse when hardness is lowered by increasing terpolymeric content of these blends. In general, it is difficult to make materials for use in, for example, a golf ball cover layer that possess good feel, high speed, high resilience, and good shear durability, and that are within a wide range of hardness. Additional compositions meeting these criteria are therefore needed.
In the past, in addition to the materials discussed above, fillers have been added to base material compositions used in the construction of golf balls. The filler generally has been added for one of two purposes: 1) as a reinforcing agent; or 2) to adjust the weight or density of a composition used in the formation of golf ball cores, intermediate layers, or covers. The prior art is replete with examples of both.
Descriptions of use of fillers reinforcing agents are found in, for example, U.S. Pat. No. 3,883,145 to Cox et al. discloses hydrated silica and barytes as reinforcing material. U.S. Pat. No. 5,759,676 to Cavallaro et al. discloses addition of glass fibers to cover material as a reinforcing agent. This also is shown in commonly-owned U.S. Pat. No. 6,012,991 to Kim et al., which discloses glass fibers used as a reinforcing agent in a golf ball intermediate layer composition. U.S. Pat. No. 4,836,552 to Puckett discloses incorporation of glass bubbles into a ball material composition to improve impact resistance.
Descriptions of fillers used to modify the density or weight of a golf ball composition include U.S. Pat. No. 1,369,868 to Worthington, which discloses the addition of wolframite to the core of a golf ball. The addition of wolframite increases the overall density of the core so that additional weight and, as a consequence, additional ball flight are obtained. U.S. Pat. No. 3,671,477 to Nesbitt describes the addition of filler material to a golf ball to control its weight without affecting its resilience. The filler used in the Nesbitt patent preferably includes 20 to 40 parts per hundred by weight of hydrated silica. U.S. Pat. No. 4,863,167 to Matsuki discloses addition of heavy fillers such as tungsten and lead to a mantle layer of a golf ball to push weight away from the core of the golf ball. The Matsuki patent also utilizes composition fillers such as zinc oxide, barium sulfate, silica and zinc carbonate to maintain correct weight proportions for the cover and core of the disclosed golf ball. U.S. Pat. No. 5,312,587 to Sullivan discloses the use of high ratio quantities of metal stearates in compositions to act as fillers without reducing C.O.R. values. The Sullivan patent states that such a use is beneficial for reducing the material costs of golf ball compositions. The Sullivan patent also points out that small amounts of zinc stearate (i.e., from 0.01 to 1.0 pph) previously had been used in the golf ball industry for facilitating the flow of ionomer resins, and that the improvements of metal stearates as a filler are only shown when the amounts used are greater than 10 pph of ionomer resin. U.S. Pat. No. 6,123,929 to Gonzenbach et al. discloses use of glass fibers, barium sulfate and metal stearates as a filler material for manipulating the density of the golf ball compositions used.
The examples discussed above generally include large amount of filler material, usually greater than 5 pph of the base composition, and often greater than 20 pph of the base composition. These large amounts are required for the filler material performs its function, either as a reinforcing agent or as a weight/density-modifying material. From another perspective, it is seen that the fillers previously have been added with the explicit purpose of altering the generally tested mechanical properties of a golf ball (i.e., C.O.R., weight, shear resistance, and spin) without regard to any change in non-mechanical properties that may occur due to the addition of the filler material.
Of the physical characteristics of a golf ball, the two most sought are high resilience and good feel. High resilience gives a ball added distance, which is particularly desired by casual golfers. However, high resilience balls (also known as distance balls) generally are considered hard golf balls and do not provide good feel for pitch shots and putting. A golf ball having what is called good feel typically is softer than its distance counterpart. This gives the golfer more confidence to control the distance of a putt or a pitch shot, but it offers less distance for long shots. The perceived feel of a ball is determined by more, however, than its compression and resilience characteristics. When determining the feel of a golf ball, most avid golfers, from casual to professional, are sensitive to the sound of the ball when struck. A louder, higher-pitched sound is associated with a hard, high resilience ball, while a softer, lower-pitched sound is associated with a soft ball.
Testing of sound characteristics when struck has been performed on golf balls. A particular family of patents discloses frequencies of specific golf balls materials. These patents include U.S. Pat. Nos. 5,971,870, 6,425,833, 6,142,866 and 6,152,835, collectively assigned to Spalding Sports Worldwide, Inc. These patents discloses a golf ball made from a material, such that the golf ball has a primary minimum value in a frequency range of 3100 Hz or less. An explanation follows of what causes the audible sound emitted from a golf ball when it is struck by a golf club and how that sound is measured.
A golf ball, when it is struck, is contracted along a primary diameter from the point tangent to where the golf ball was struck. The golf ball has a fixed circumference, and any contraction along the primary diameter causes a secondary diameter, perpendicular to the primary diameter, to elongate as it compensates for the narrowing of the primary diameter. Though this happens in three dimensions, it can be thought of as horizontal line X and vertical line Y, wherein X is synonymous with the primary diameter and Y is synonymous with the secondary diameter. The sum of their lengths remains equal, thus, an extension of one necessitates a narrowing of the other, and vice versa. The resiliency of the material causes the now-narrowed primary diameter to expand back to and beyond its original length, while the secondary diameter contracts to a length less than its original length. The deformation of the golf ball diameters between extension and contraction defines an oscillation (or pressure pulse) that vibrates against air molecules. The vibration of the air molecules is, in effect, the sound that we hear. The contraction and extension of the golf ball is greatest along the primary diameter and second diameters, because the primary diameter is tangent to where the ball was struck. Because the primary and secondary diameters oscillate more than other diameters of the golf ball, the oscillation of the primary and secondary diameters define the first acoustic mode which generates the most audible pressure pulse. In the above-mentioned Spalding patents, this first acoustic mode is called the primary value. The purpose of the inventions disclosed in these patents is to produce a cover material having a specific first acoustic mode having a frequency lower than 3100 kilohertz however, in these patents, no effort was made to alter either the decibel level or the frequency of the materials produced.
Because a golf ball is solid, it cannot oscillate only between two diameters or even two perpendicular planes. The solid nature of the ball causes additional oscillations on planes that are not coplanar with either the primary or secondary diameters. Additional acoustic modes are caused by oscillations along other diameters and include a great number of diameters. The second acoustic mode includes elongation and contraction along three diameters that intersect each other, the third acoustic mode includes four diameters and so on. While theoretically there is no limit to the number of acoustics modes, as spheres have an infinite number of diameters, there is a limit to which we can pick out the nodes with sound listening equipment. As the energy input increases, higher order acoustic modes are excited. Generally, the oscillations of these acoustic modes are small and their frequencies are too high for the human ear to detect. For that reason, it is generally the first, second, and sometimes third, acoustic modes that are the most important acoustic modes. Also, altering the frequency of the first acoustic mode will alter the frequency of the remaining acoustic modes. Thus, lowering the frequency of the first acoustic mode will lower the frequency of the second and third acoustic modes, so that the overall sound detected has a lower frequency.
The frequency of the golf ball is most important to altering the perceived sound of the ball when struck when putting or making short shots, such as pitching onto a green. Thhis is because a golf ball struck with a longer club, such a driver, does not oscillate as much as the head of the club which struck the ball. For that reason, when a golfer strikes a golf ball with a driver, the driver primarily provides the sound that is heard, and little is given to the golfer in the way of soft or hard impressions relating to the ball. Conversely, when a golfer strikes a ball with a putter, the mass of the putter and ease of the stroke cause little oscillation in the putter and therefore the “click” of the golf ball is heard.
Another way to measure sound with respect to golf ball constructions and materials is to primarily rely on decibel levels. The decibel level includes all of the acoustic modes and is a function of how much sound is emitted from the material when it is struck. Decibels are converted from Pascals, which indicate the magnitude and duration of the pressure pulse associated with the sound. A ball emitting a smaller pressure pulse (lower Pascal output) will give the impression of a softer feeling. This is true even if measurements of the C.O.R. indicate that the material properties of the golf ball have remained essentially the same.
Golf balls having a high pitch or high acoustic output are viewed as too hard, while balls having a low pitch or low acoustic output are perceived as a ball having a short flight distance. This perception holds true regardless of the actual mechanical properties of the golf ball in question. In view of this problem and the ones stated above, it is apparent that a method to adjust the frequency or Pascal output for golf balls, while retaining the C.O.R. of the golf balls, as well as the golf balls including such features, is needed. This will allow the manufacturer to adjust the sound of the golf ball so that it is tuned to the satisfaction of a golfer, while retaining the mechanical properties (i.e., C.O.R., resilience) of the ball. The present invention fulfills this need and provides further related advantages.